The present invention relates generally to a program-controlled shutter adapted to attain the optimum exposure depending upon the brightness of a subject, and more particularly a program-controlled shutter of the type in which a plurality of shutter blades which also function as the aperture setting blades are driven stepwise by a step-motor so that the optimum aperture may be set for the optimum time interval depending upon the brightness of a subject.
The program-controlled shutters of the type described above have been disclosed, for instance, in U.S. Pat. No. 3,882,522, U.S. Pat. No. 3,813,680 and the copending applications filed by the same inventors, Ser. Nos. 550,367 filed on Feb. 18th, 1975, 567,231 filed on Apr. 11th, 1975, and 569,006 filed on Apr. 17th, 1975. These prior art program-controlled shutters were devised in order to overcome the defects encountered in the prior art program-controlled shutters of the type using a moving coil or governor for controlling the shutter or aperture setting blades or diaphragms. That is, the prior art program-controlled shutters use a step-motor for driving stepwise the shutter or aperture setting blades or diaphragms. The common underlying principle of these program-controlled shutters is that the step-motor is rotated stepwise in response to the number of pulses which in turn are generated by a pulse generator in response to the brightness of a subject, whereby the aperture and shutter speed may be selected. Therefore if the shutter or aperture setting blades or diaphragms are not coupled to the step-motor with suitable mechanical coupling means which permits the smooth transmission of the driving power from the step-motor to the shutter or aperture setting blades or diaphragms or when the step-motor's response to the driving pulse is slow, the number of stepwise angular rotations of the steo-motor is different from the number of driving pulses applied thereto, resulting in the erratic exposure; that is, under- or over-exposure. Furthermore, since the prior art program-controlled shutters of the type described above must incorporate a control circuit for controlling the pulse generator, they are relatively complex in construction.